Model plaster



" .0a COMPOSITIONS,

COATING R PLASTIC. Examm' Patented Mar. 14, 1933 1,901,056

0 UNITED STATES .PA'FENT OFFICE.

WILBUR S. RANDEL, OF FABNAMS, MASSACHUSETTS, ASSIGNOR T0 UNITED STATESGYPSUM COMPANY, OF CHICAGO, ILLINOIS, A CORPORATION OF ILLINOIS MODELPLASTER No Drawing. Application filed January 13, 1932. Serial No.586,377.

This invention relates to cementitious its setting time is approximatelythat of ormaterials, and has reference more partic, dinary plaster ofParis, viz., to minlarly to cementitious materials made of hi utes. Thecompressive strength of this strength calcined gypsum and suitable};product Wlll range from 2,500 to 6,000

5 use in the making of models. pounds per square inch and it has apouring In the use of die sinking machines in the consistency of lessthan 50 ccs. This high automobile industry, it is a practice to caststrength calcined gypsum which I shall in models of fenders .and otherparts which are the future designate as al ha sum is to be reproduced insteel dies by die sinking manufactured by treating lumps oi gypsum 10machines. In these models, it is essential rock, having a diameter of to2", in a that accurate reproduction of dimensions be closed containerwith steam at 17 to 20 secured in the steel die. It is also desirablepounds gauge pressure. Steam at 15 to 50 to have the model plaster usedfor making pounds pressure may be used, but outside of the model setwithin an hour after the model the preferred range of 17-20 poundsgauge,

15 is cast and to have a suificient strength so the strength and qualityof the resulting that it can be handled without breaking, and productdiminishes. This steam calcination a ffi i nt hardn o that th f l r ofth is carried on for 5 to 7 hours with a constant die sinking machinewill leave no impress eam pressure, and with a constant withi as it travl Over th od l, thu in u drawal of water of condensation obtained 20 ingaccuracy in reproduction of dimensions fr m the heating steam and alsofrom the f th t l di I th ti f very la expelled water ofcrystallization. After calmodels, the necessity for an extremely lowrlat on, the product is dried while mainexpansion is apparent when it isconsidered m ng t a temperature under 400 F. and th t an expansion f0,15%, th mal the product is ground so that 86% or more 25 ansion ofordinary molding plaster, is in a of same passes through a 100 meshscreen. 5 foot length equivalent to lineal di- The crystals of ordinaryfirst settle, calmensional change. Models 5 feet in length cined stuccoas seen under crossed nicols in are quite common. a petrographicmicroscope, are seen to be very An object of this invention, therefore,is fine and needle-like, these crystals being most- 30 to provide amolding plaster for the making ly less than microns in diameter. Thecrys- 30 of lar e models, which will be quick setting tals of themproved alpha gypsum, under and will have suflicient strength to bereadily the same microscope and at the same time handled, and havesuflicient hardness so that magnification, appear rather short, thickand the feeler of a die sinking machine will leave Well formed. Thecrystals of alpha gypsum 35 no impression as it travels over the model.are practically pure calcium sulphate hemi- Another object of theinvention is to prohydrate, and under the crossed nicols of this vide amodel plaster which will have a very microscope, they appear brilliantlycolored, low expansion on setting, thus insuring acwhile little or nocolor is visible in the case curate dimensions of the steel die; also toof the tiny, needle-like crystals of ordinary 4 improve cementitiousmaterials in other replaster of Paris. I

spects hereinafter specified and claimed. The present invention dealsmore particu- In the co ending application of Randel larly with acomposition containing this al; and Daily, erial No. 384 343 filedAugust 9, ha sum together with a substantial pro- -57 1929, andentitled, High strength calcined portion of Portland cement to reducethe ex- 45 gypsum, a new, high strength calcined pansion of thecomposltion upon setting to gypsum is described, together with itsmethsubstantially zero. Analphagypw-P ort; 0d of manufacture. This highstrength calland cement composition consisting of 65 cined gypsum hasunique properties in that parts afpha gypsum and 35 parts Portland itscompressive and tensile strengths equal cement has a setting expansionof FY0320 to 50 or exceed that of Portland cement, whereas 0.05%. Theollowing table gives the properties of straight alpha gypsum, Keenescement, ordinary molding plaster, a mixture of parts of molding plasterand 35 parts of Portland cement, and a mixture of 65 parts of alphagypsum and 35 parts of Portland cement:

In comparing these materials, it will be noted that only one mixture,the molding plaster-Portland cement mixture, has an expansion comparableto that of the alpha gypsum-Portland cement composition although stilltwice as great. This molding plaster-Portland cement mixture has onedesirable property of quick setting, but has two very undesirablefeatures, viz., low strength and poor hardness. The alpha gypsum hassatisfactory strength, hardness and set, but its expansion is much' toohigh. The'Keenes cement has satisfactory strength and hardness, but theexpansion is rather high and the set is quite long,Keenes cement casts,in general, not being usable until the day following the castingoperation. Straight molding plaster has a quick set, rather highexpansion and low strength and hardness.

The amount of Portland cement used in the composition is dependent uponthe expansion characteristics desired. The expansion of neat alphagypsum is about 0.25%. The expansion is progressively decreased byincreasing amounts of Portland cement, until a minimum expansion ofabout 0.02 to 0.03 is reached at 50% Portland cement. Proportions ofcement above 50% cause a progressive weakening of the cast. It isdesirable, therefore, to use a minimum amount of Portland cementconsistent with obtaining the desirable low expansion so that maximumstrength and hardness, due to the alpha gypsum constituent, areobtained. One composition consisting of 65% alpha gypsum and 35%Portland cement has been found to work out very well in practice. On atest bar 4 feet in length, the expansion of this mix is so slight thatit is hardly measurable with a steel rule, being to the extent of 1/64or less. In comparison, ordinary molding plaster cast in a 4 foot testbar shows a very discernible expansion which measures about in total bythe steel rule. The percentage of alpha gypsum may range from 50-80 andPortland cement from OtW of hydraulic cement may be substituted forPortland cement, but a white Portland cement is preferred.

I would state in conclusion that while the illustrated examplesconstitute a practical when mixed with water.

WILBUR S. RANDEL.

